The present invention relates to a method and apparatus for hydraulic isolation determination of oilfield casings. More specifically, the interfaces between the various materials present in the borehole are interrogated using ultrasonic energies to characterize the materials and the bonds formed between them.
In a well completion, a string of casing or pipe is set in a wellbore and a fill material referred to as cement is forced into the annulus between the casing and the earth formation. The primary purpose of the cement is to separate oil and gas producing layers from each other and from water bearing strata. If cement fails to provide isolation of one zone from another, fluids under pressure may migrate from one zone to another, reducing production efficiency. Migration of water, in particular, produces undesirable water cutting of a producing zone and can possibly render a well non-commercial. Also, migration of hydrocarbons into aquifers is environmentally and economically undesirable. It is critical to determine whether the cement is performing its function, i.e., whether the hydrocarbon reservoirs are hydraulically secure. The term "good cement" indicates the adequate separation of zones by the cement, preventing fluid migration between the zones.
Cement failures occur in a variety of ways. For example, a complete absence of cement between the casing and the earth formation can occur. This is characterized as a gross cement failure and leads to rapid communication between zones intended to be isolated. Another type of failure arises when channeling occurs within the cement annulus, between the casing and the formation. There are three commonly occurring types of channels. First, a channel which contacts the casing is referred to as a "near channel". Second, a channel which does not contact the casing is referred to as a "far channel" or a "buried-channel". For a buried channel, the region between the channel and the casing is usually cement. And third, a channel occupying the entire space between the casing and the formation is referred to as either a "full channel" or a "traditional channel". All these channels described above are filled with fluids such as mud or gas and all are potential threats to hydraulic isolation.
Another condition which occurs, but which is not generally viewed as a cement failure, is known as micro-annulus. This condition occurs when the cement that has filled the annulus is not properly bonded to the casing resulting in a very narrow fluid-filled annulus immediately outside the casing. This annulus is very small and does not affect fluid communication between layers effectively preserving the hydraulic security function of the cement.
A completed well includes a number of interfaces at the junctures of the differing materials within the weilbore. A first interface exists at the juncture of the fluid in the casing and the casing itself. The casing is referred to as a first material and is typically made of steel. A second interface is formed between the casing and a second material adjacent to the exterior of the casing. If cement is properly placed between the casing and the formation, providing hydraulic isolation, the second interface exists between the casing (first material) and the cement (second material). Further, a third interface exists between the cement and a third material which is the earth formation.
Imperfect cementing operations can result in a variety of interface conditions. A channel contacting the casing results in the second interface being between the casing (first material) and a fluid (second material). In this case, the third interface is formed between a fluid (second material) and the earth formation (third material) where a full channel exists. Alternatively, the third interface is formed between a fluid (second material) and the cement (third material) where a near channel exists. A channel not contacting the casing, results in the second interface being between the casing (first material) and the cement (second material) and the third interface being between the cement (second material) and a fluid (third material). Existence of an interface at the juncture of cement and fluid causes a potential lack of hydraulic isolation.
The problem of investigating the fill material or cement outside a casing with a tool located inside the casing has lead to a variety of cement evaluation techniques using acoustic energy. These techniques can be categorized into sonic cement evaluation (SCE) and ultrasonic cement evaluation (UCE).
Current sonic cement evaluation can be divided into two distinct categories. The first evaluates the Cement Bond Index (CBI) which attempts to measure the percentage of the circumference of the cement adhering to the casing. The second generates a variable density log which qualitatively evaluates the cement fill in the annulus by identifying a head wave generated by a compressional wave in the formation. Both sonic techniques use non-directional or slightly directional sources and receivers and depend on energy propagation essentially parallel to the surfaces of the casing.
One SCE technique is described in U.S. Pat. No. 3,401,773 to Synott, III and assigned to Schlumberger Technology Surveying Corp. Synott describes a cement logging technique using a tool employing a conventional, longitudinally spaced, sonic transmitter and receiver. The signal traveling through the casing is processed and a portion of the signal affected by the presence or absence of cement is extracted. The extracted segment is interrogated to provide a measurement of its energy as an indication of the presence or absence of cement outside the casing. This technique provides useful information about cement defects at the second interface.
Current ultrasonic cement evaluation also concentrates on the second interface to determine whether cement or mud is adjacent to the casing in an annulus between the casing and the earth formation. A number of known techniques use a pulse-echo method. A single transducer transmits energy into the casing at near-normal incidence and receives echoes. The signal excites a resonance within the casing and the properties of the resonance are measured and interpreted to indicate whether cement or undisplaced mud lies just outside the casing. The main limitation of such techniques is that they concentrate on the second interface ignoring the effects of the third or further interfaces.
Ultrasonic pulse-echo techniques for determining the thickness of materials have been extensively proposed in the art. For example, U.S. Pat. No. 2,538,114 to Mason describes an apparatus for measuring the thickness of a material by noting its resonance frequency whenmaterial is irradiated with ultrasonic energy. In U.S. Pat. No. 4,003,244 to O'Brien, et al., the thickness of a material is measured by employing a pulse echo technique.
U.S. Pat. No. 4,255,798 to Havira describes methods and apparatuses for acoustically investigating a casing in a borehole to determine whether cement is present just outside the casing. Casing thickness is also determined. The techniques employ an acoustic pulse source having a frequency spectrum selected to excite a thickness resonance in the insonified portion of the casing. The thickness resonance exists as acoustic reverberations between the inner and outer walls of the casing, i.e., trapped energy. The duration of the reverberations depends on the rate of acoustic energy leaking into adjacent media. The acoustic return from the casing can be thought of in two distinct portions. The first appears as a large amplitude pulse which represents the energy reflected from the first fluid-steel interface, i.e., the inside surface of the casing. The second appears as a decaying resonance which represents the reverberating energy trapped within the casing that has leaked back into the fluid within the casing. The received acoustic pulse is then processed to determine casing thickness or to evaluate the quality of the cement bond to the casing.